... depends on your local climate. If you live somewhere perpetually warm, a summer suit would be a good next choice. Linen or fresco-style light wool, in a lighter colour. That might be a good choice generally as it's easier to wear an extra t-shrit or long undies to make a summer suit work for the fall, but a thick suit can't work in the heat no matter what.

Mid-grey or RAF blue flannel around 13oz. Mid-grey could have a very subtle windowpane. Or, if you're feeling slightly more daring, a nice, subdued flannel PoW (predominantly grey or tan, I'd say). Or you could do a blazersuit in a blue hopsack of sorts.